(1.) Field of the Invention
The present invention relates to an irreversible circuit element having an irreversible transmission character relative to a high-frequency signal. More specifically, the present invention relates to an irreversible circuit element for use in a mobile communication system such as a portable telephone, generally referred to as an isolator and a circulator.
(2.) Description of the Related Art
Users of a portable telephone and a mobile telephone have recently increased in number with an increase of their coverage area and a smaller size of an equipment for them. One main component of the portable telephone, the mobile telephone and the like is an irreversible circuit element. The irreversible circuit element allows a signal to be passed in a transmission direction alone, and it prevents the signal from being transmitted in an opposite direction. The irreversible circuit element is referred to as a circulator and an isolator.
When the irreversible circuit element is inserted into its circuit system, a loss of a signal power to be transmitted occurs. Accordingly, the irreversible circuit element is heated, and battery consumption is increased. Many attempts have been made to efficiently operate the irreversible circuit element without losing a high-frequency signal and to reduce a signal power loss.
For example, in Japanese Patent Laid-Open No. 7-106809/1995, a loop strip line to be resistance-connected has a different width from the width of other loop strip lines whereby an impedance mismatch which is caused by a resistance connection is corrected. That is, a design of a central conductive material attempts to solve the above problem.
Furthermore, in Japanese Patent Laid-Open No. 6-164211/1994, an auxiliary ferrite is disposed whereby the design of a magnetic circuit attempts to solve the problem.
Furthermore, in Japanese Patent Laid-Open No. 6-204712/1994, an earth potential of a yoke achieves a reduction of power loss. The yoke is used to form an outermost of the irreversible circuit element. The yoke holds many components which are incorporated in the irreversible circuit element so that they may be positioned in place. In addition, the yoke serves not only as a case for protecting the components but also as one part of the magnetic circuit which the whole irreversible circuit element is composed of. The yoke also has an electric shield effect so that it may reduce an interference between an inside and an outside of the irreversible circuit element. When the power loss is high within the irreversible circuit element, the yoke plays an important role in efficiently dissipating the generated heat.
One of the most important functions is to efficiently construct the magnetic circuit having an excellent uniformity. Thus, a nickel-plated iron plate is generally used for a material of the yoke. Furthermore, the iron is silvered and the silvered iron is then coated with an insulating resin. The resultant is proposed as the lower yoke.
The above-described conventional magnetic yoke for the irreversible circuit element employs the nickel and the iron which have an intermediate electric conductivity as a metal material. Accordingly, the materials do not always have an excellent signal transmission efficiency for a high-frequency electric signal which is sensitive to the electric conductivity. More effectively, the base-metal iron is improved so that it may be replaced by the high-conductivity material. However, a performance for the magnetic circuit might be deteriorated.